The present invention is concerned with a Petri dish, especially for culturing micro-organisms having a bottom dish and a cover and providing means to control gas exchange between the interior of the Petri dish and the atmosphere.
In microbiological techniques, Petri dishes of various sizes and of differing diameter are used for culturing micro-organisms. Petri dishes with diameters of from 50 to 150 mm. and with edge heights of 14 to 20 mm. are conventional. The materials used for making Petri dishes include glass, synthetic resins and, less frequently, corrosion-resistant metals, transparent synthetic resin materials preferably being used.
Two types of Petri dishes are known, namely, those the covers of which have projections on their inner surface and those the covers of which are free of projections on the inner surfaces. The usual three small, about 1 to 2 mm. high nub-like projections lie on the flat edge of the lower dish and prevent the cover from lying closely upon the edge of the lower dish so that a slit is formed through which gas exchange is possible with the inner space of the Petri dish.
After pouring in a nutrient substrate, especially a gel-like nutrient substrate, gas exchange is important for the culturing process for drying the surface, for the gas exchange carbon dioxide:oxygen or of other volatile metabolic products and, in the case of anaerobic culture processes, for the removal of oxygen or of the oxygen-containing atmosphere or for the replacement thereof.
In the case of Petri dishes which do not have projections, so that the cover lies directly on the rim of the lower dish and gives a tight closure of the Petri dish, because of the absence of gas exchange, there are frequently difficulties in culturing micro-organisms. On the other hand, when the cover lies directly on the rim of the lower dish, drying out of the nutrient substrates or of the investigation material is very much lower in comparison with Petri dishes with projections so that, inter alia, the nutrient substrates in the Petri dishes can be kept much longer and remain usable for longer periods of time.
In the following, there is explained, with reference to a CSP-agar storage-stability test, the action of Petri dishes with and without projections on the bottom part of the cover.
The drying out of CSP-agar in Petri dishes was tested under various conditions, using Petri dishes with the dimensions of 90.times.20 mm. made of polystyrene with and without nub-like projections.
18.5 g. amounts of CSP-agar were poured into the Petri dishes and then kept for 1 day at ambient temperature prior to commencement of the experiment.
The agar Petri dishes with and without projections were then stored for 13 days under various conditions:
(1) unpacked at 4.degree. C., 20.degree. C. and 37.degree. C.; PA0 (2) batches of 5 dishes enclosed in Frappan, then packed in batches of 10 dishes in an MD-carton at 4.degree. C., 20.degree. C. and 37.degree. C.; PA0 (3) batches of 5 dishes in a Cellophane bag and then, again in batches of 10 dishes, in an MD-carton at 4.degree. C., 20.degree. C. and 37.degree. C.
The weight of the agar was determined by weighing at the commencement and at the end of the testing.
The results obtained are summarised in the following Table:
______________________________________ decrease of the agar weight in extrapolated temper- projec- 13 days to 50% loss ature tions packing % loss (days) ______________________________________ 4.degree. C. + - 2.77 235 + Frappan + carton 0.69 942 + Cellophane + 2.58 252 carton - - 1.22 532 - Frappan + carton 0.15 4333 - Cellophane + 1.01 643 carton 20.degree. C. + - 21.71 30 + Frappan + carton 6.33 103 + Cellophane + 7.40 88 carton - - 11.61 56 - Frappan + carton 2.22 293 - Cellophane + 5.89 110 carton 37.degree. C. + - 62.70 10 + Frappan + carton 8.32 78 + Cellophane + 28.59 23 carton - - 34.45 19 - Frappan + carton 6.09 107 - Cellophane + 16.61 39 carton ______________________________________
The above Table demonstrates the drying out storage stability of an agar nutrient media (CSP) in standard Petri dishes with and without projections under various conditions: temperatures of 4.degree. C., 20.degree. C. and 37.degree. C. without packing and with packing in Frappan + carton and Cellophane + carton.
Extrapolation of the drying out losses to a theoretical limiting concentration of usability of 50% drying out loss gives, as can be seen, comparable values.
Not only the temperature conditions of the storage but also the packing and the presence or absence of projections on the Petri dishes have a decisive influence on the extent of the drying out of the agar.
With regard to the storage temperature, the low temperature of 4.degree. gives the longest periods of storage stability. Unpacked Petri dishes have a theoretical storage period of 235 days at 4.degree. C., 30 days at 20.degree. C. and only 10 days at 37.degree. C.
By omitting the projections, these times are lengthened to about twice as much, namely, 532, 56 and 19 days, respectively.
The packing brings about a further prolongation of the storage stability. This is greater in the case of Frappan-packed Petri dishes than in the case of Cellophane packing.
The best results were obtained, in each temperature group, in the case of nutrient media in Petri dishes without projections, enclosed in Frappan and packed into an MD carton. In this case, the theoretical storage stability period, even at 20.degree. C., was still 293 days.
Water of condensation did not appear to any appreciable extent during the experimental period.
From these experiments, it follows that the storage of prepared nutrient media is mainly made difficult by two factors: drying out of nutrient agar in Petri dishes which are not firmly closed and the formation of water of condensation.
Drying out mainly takes place due to air exchange between the lower dish and the cover lying loosely thereupon. This possibility of air exchange is favoured by the presence of the three nub-like projections on the lower side of the cover, which serve as distance pieces and thus give rise to a narrow circular air gap. This air gap for the better aeration of the cultures during culturing and also for reducing the formation of water of condensation on the inner surface of the lid is frequently very advantageous.
However, under certain circumstances, for example comparatively long culturing times because of poor growth, it is only possible to use dishes with covers without projections since otherwise the drying out would have a disturbing effect.
The above-described experiments clearly demonstrate the influence of the projections acting as distance pieces on the drying out of the nutrient substrate.
It is an object of the present invention to avoid the use of two different types of Petri dishes and to provide a single Petri dish which enables not only a distancing by means of projections but also, if desired, a tight closure.